GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,
THEMIS observations of an earthward propagating1
dipolarization front2
A. Runov,1
V. Angelopoulos,1
M. I. Sitnov,2
V. A. Sergeev,3
J. Bonnell,4
J. P. McFadden,4
D. Larson,4
K.-H. Glassmeier,5,6
and U. Auster,5
1Institute of Geophysics and Planetary
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X - 2 RUNOV ET AL.: DIPOLARIZATION FRONT
This paper presents an analysis of THEMIS observations of a dipolariza-3
tion front (sharp large-amplitude increase of the Z-component of the mag-4
netic field). The front was detected by four of the five probes, first by the5
outermost probe (P1) at X=-20.1RE then by P2 at X=-16.7RE, and finally6
by the P3/P4 pair at X=-11.0RE. Thus the dipolarization front, born tail-7
ward of -20RE, propagated earthward at a velocity of 300 km/s. The front8
thickness was found to be comparable to the ion inertial length. Compar-9
ison between observations and simulations allows us to interpret the front10
Physics, University of California, Los
Angeles, CA, USA
2Applied Physics Laboratory, Johns
Hopkins University, Laurel, MD, USA
3St. Petersburg State University, St.
Petersburg, 198504, Russia
4Space Science Laboratory, University of
California, Berkeley, CA, USA
5Institute of Geophysics and
Extraterrestrial Physics, Technical
University Braunschweig, Germany
6Max Planck Institute for Solar System
Reseach, Katlenburg-Lindau, Germany
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RUNOV ET AL.: DIPOLARIZATION FRONT X - 3
as the leading edge of a flow burst formed by an impulse of magnetic recon-11
nection in the midtail.12
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1. Introduction
Spatially and temporally localized dipolarizations (increases of the magnetic field ele-13
vation angle) are often observed in the near-Earth as well as in the mid-tail plasma sheet14
during bursty bulk flow (BBF) events. Recent superposed epoch analyses based on Geotail15
data reveal similarity in earthward BBF-related variations of the north-south magnetic16
field component (Bz), ion density and temperature observed at -31< X
RUNOV ET AL.: DIPOLARIZATION FRONT X - 5
northward magnetic field and fast propagation of that front structure over a long distance33
(several RE), may indeed be explained by transient reconnection in the magnetotail.34
In this paper we report on observations of a DF by five THEMIS spacecraft (probes),35
situated in the near-equatorial plasma sheet at distances of -20< X
X - 6 RUNOV ET AL.: DIPOLARIZATION FRONT
Between 0751 and 0754 UT similar front-like variations of the Bz field were detected54
consecutively by P1, P2, and P3/P4 probes (Figure 1 c) with a more gradual buildup of55
Bz at P5, which was located 1RE southward of P3/P4. Assuming planar front, the timing56
of the DF at P1 (0751:26UT) and P2 (0752:35UT) suggests earthward propagation at a57
velocity of 330 km/s. Timing at P2 and P4 (0754:10UT) results in 360 km/s. Thus, the58
DF propagates from -20 to -11RE without deceleration.59
Figure 2 summarizes observations at P1 and P2 during 2.5 min around the DF (0751:0060
- 0752:30UT and 0752:00 - 0753:30UT, respectively). P1 was in the northern half of the61
plasma sheet close to the neutral sheet, while P2 was mainly in the southern half and in62
the neutral sheet. Between 0751:15 and 0751:30UT, P1/FGM detected bipolar (negative63
to positive) variations of Bx (blue trace) and Bz components (red trace), accompanied by64
a positive variations of By. Since the burst-mode survey was triggered by the variation of65
Bz, the FGM data with the highest time resolution (FGH, 128 vectors/s) are available.66
Using FGH data, the precise duration of the front passage from negative Bz peak (-5 nT)67
to the first local positive peak (20 nT) was 1.35 s, (Fig. 2 c), i.e., the DF thickness was of68
400 km. Variations of the electric field components Ey and Ex, shown in DSL (close to69
GSE) coordinates, registered by P1/EFI are consistent with the PIC simulations [Sitnov70
et al., 2009]. The time-energy spectrograms, combining high- (SST) and low-energy (ESA)71
ion and electron spectra, show rapid changes indicating ion and electron energization.72
Plasma moments reveal a drop of the plasma density and pressure accompanied by a73
gradual growth of the earthward bulk flow speed up to 1000 km/s and an increase of the74
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RUNOV ET AL.: DIPOLARIZATION FRONT X - 7
magnetic pressure. These are the distinctive features of BBFs [Angelopoulos et al., 1992]75
supported by the model of plasma bubbles [e.g., Birn et al., 2004, and references therein].76
P2, located 3.4RE earthward of P1, detected the bipolar (small-amplitude negative then77
sharp high-amplitude positive) variations in |Bx| and Bz between 0751:25 and 0752:35UT.78
They were associated with a positive By variation, increase of Ex and Ey, rapid changes79
in particle ET spectra, a drop of the ion density and increase of the earthward plasma80
flow. Pp increased while Pm decreased during the negative Bz variation (this effect is more81
pronounced at P2 than at P1). Pp rapidly decreased while Pm quickly rose on the DF.82
The peak-to-peak (1.5 and 31 nT) front propagation time was estimated using FGH data83
(Fig. 2 d) to be 1.70 s, i.e., the DF was 500 km thick.84
Figure 3 shows DF observations at P3 and P4, situated at the same X and Z and85
separated in 1RE in YGSM . At 0753:30UT, both P3 and P4 were in the southern half of86
the plasma sheet at Bx=-25 and -22 nT, respectively, detecting Bz=3nT. At 0753:50UT,87
P3 started to detect a decrease of |Bx| and |By|, without significant variations in Bz. A88
sharp decrease of Bz down to zero was observed between 0754:06.0 and 0754:06.5UT.89
Both |Bx| and |By| also reached local minima at that short time interval, which results90
|B|=2.3 nT. Bz reached the local maximum (19 nT) at 0754:08.1UT. The DF passing time91
(1.6 s, Fig. 3 c) corresponds to DF thickness of 500 km. P4 started to detect a decrease of92
Bz and an increase of By at 0753:57UT. A local minimum of Bz (-8 nT) was detected at93
0754:10.3UT, and a local Bz maximum (26 nT) was at 0754:12.0UT (1.7 s front passage94
duration, 500 km thickness). The magnetic field variations and the signatures in particle95
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spectra and moments detected by P3 and P4 are similar to those, detected earlier by P196
and P2, which suggests the probes encountered the same spatial structure.97
Observations made by P5, located at the same X and Y as P4, but 1RE southward,98
between 0752:30 and 0755:00UT are summarized in Figure 4. Although P5 observed99
more gradual dipolarization (see also Fig 1), the signatures in the spectra, moments and100
magnetic pressure at P5 were similar to those at the other probes. They started earlier101
compared to P3/P4, presumably because P5 was much farther from the neutral plane102
(Bx=-36 nT at 0752:30UT). Lag time between increases of Pm at P1 and P5 (92 s) gives103
a propagation velocity of 400 km/s.104
Minimum Variance Analysis (MVA) [Sonnerup and Scheible, 1998] was applied to the105
magnetic field time series at four probes (P1 - P4) detecting the DF to determine the front106
orientation. The MVA results are summarized in Table 1. The three MVA eigenvectors:107
R1, R2, and R3, corresponding to the three eigenvalues: λ1, λ2, and λ3 define maximum,108
intermediate, and minimum variance directions in the GSM coordinates, respectively. To109
define the direction unambiguously, the X-component of the minimum variance direction110
was set to be positive (earthward) in accord with the propagation direction defined from111
timing. R3 may be interpreted as the front-normal vector. At P1 and P2, the normals112
were close to the X direction, indicating a boundary in the Y Z plane. The bulk velocity113
directions were close to the normals (Figure 4 b). At P3 and P4, the XZ projections of114
normals were consistent, while XY projections were almost orthogonal. The normal at115
P3 indicates a tilt of the front in the XZ plane. Projections of bulk velocity directions at116
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P3 and P4 were similar. We did not apply MVA for P5 data in view of the rather gradual117
variations of the magnetic field there.118
3. Discussion and Summary
Multi-point observations by the five THEMIS probes during 0751 - 0755UT on Feb.119
27, 2009 for the first time provide unambiguous evidence of earthward propagation of the120
dipolarization front (DF), which was born tailward of 20RE, up to 11RE at the velocity121
of 300 km/s. The DF was found to be a thin boundary, separating plasmas with different122
properties. The DF thickness was 400 - 500 km, which is comparable with the ion inertial123
length (di ∼300 km for N=0.5 cm−3) and smaller than the 10 keV-proton gyroradius in124
Bz=15nT (1000 km). Thus, observations reveal an example of the space plasma self-125
organization, when a micro-scale structure, with thickness less than the ion gyroradius126
implying different motions of ions and electrons, remains structurally stable during its127
propagation over a macroscopic distance of about 10RE.128
P5, located 1 RE southward of P4, observed signatures of the DF about a minute129
prior to P4. It may be explained by faster propagation of disturbances at the plasma130
sheet periphery where the Alfvén speed is larger [e.g., Krauss-Varban and Karimabadi ,131
2003]. Another cause of the relatively early DF signal detection at P5 is the sharply132
curved shape of the DF flux tube [Nakamura et al., 2005], which is also seen in MHD133
simulations of plasma bubbles [Birn et al., 2004]. Figure 4 b summarizes the observation134
and interpretation scheme. The MVA normal directions suggest that the dipolarization135
region was deflected dawnward, similar to earlier observations [Nakamura et al., 2002], so136
that P4 was eventually found on its dusk-side edge.137
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The observations show that the quick southward variation observed ahead of the DF138
is a spatial structure associated with the propagating dipolarization front. The observed139
spatial scales of the DF and the Bz dip (∼ di, similar to those resulting from the PIC140
simulations [Sitnov et al., 2009]) suggest their relation to the Hall-type currents due to141
ion-electron decoupling on the front, resulting in Ex observed in the simulations and in the142
data. The ∼1 s-long Bz dips down to negative values were preceded by a longer decrease of143
magnetic pressure with a simultaneous increase of plasma pressure: a diamagnetic effect144
due to plasma compression ahead of the moving dipolarization front.145
To conclude, a major conjunction of the THEMIS probes along the magnetotail provided146
the unique evidence of DFs as boundaries with a micro-scale thickness propagating over147
a macro-scale distance of 10RE at a velocity comparable to the local Alfven speed. The148
analysis of such thin boundaries requires a kinetic approach. The agreement with PIC149
simulations [Sitnov et al., 2009] suggests transient magnetic reconnection as the most150
plausible source of DFs in the magnetotail.151
Acknowledgments. We acknowledge NASA contracts NAS5-02099 and NNX08AD85G,152
the German Ministry for Economy and Technology and the German Center for Aviation153
and Space (DLR), contract 50 OC 0302. The OMNI data are available on CDAWeb. We154
thank P.L. Pritchett and L. Lyons for discussions, B. Kerr, P. Cruce, and A. Prentice for155
help with software and edition.156
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Figure 1. THEMIS sc positions in XZ (a) and XZ (b) GSM planes and c) time series
of Bz (GSM) at all five probes (P1 - P5).
Figure 2. Summary of THEMIS P1 (a) and P2 (b) observations during 0750:30
- 0755:00UT. For each probe from top to bottom: X,Y ,and Z GSM components of
the magnetic field; X and Y DSL (Y component was 180◦ rotated to point duskward)
components of the electric field (spin resolution); Ion energy-time spectrogram combining
SST and ESA ions (a blank stripe indicates the energy gap between the two instrument
ranges); Electron (SST and ESA) time-energy spectrogram; ion number density X, Y ,
and Z GSM components of the ion bulk velocity calculated with ESA and SST inputs;
Magnetic (Pm) and plasma (Pp) pressures. c) and d): GSM B (128 vectors/s) during 15 s
around the dipolarization front at P1 and P2, respectively.
Figure 3. Summary of THEMIS P3 and P4 observations during 0750:30 - 0755:00UT.
The same format as in Fig. 2.
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Figure 4. a) Summary of THEMIS P5 observations between 0752:30 and 0755:00UT.
The same format as in Fig. 2. b) An interpretation scheme. Black arrows are projections
of MVA normals (R1) onto XY (upper panel) and XZ (bottom panel) GSM planes; blue
arrows are projections of bulk velocity directions to the XY GSM plane. Gray segments
represent a flux tube with the enhanced magnetic flux populated by hot and tenuous
plasma. Times of DF crossing by each probe are shown.
sc UT λ1, λ2, λ3, R1 R2 R3
P1 0751:26 76.5, 6.02, 0.13 0.37,-0.16, 0.91 -0.29,-0.95,-0.05 0.88,-0.25,-0.40
P2 0752:35 107.0, 8.93, 0.17 0.35,-0.26, 0.90 -0.50,-0.86,-0.05 0.79,-0.43,-0.43
P3 0754:07 102.3, 4.64, 0.72 0.83,0,-0.08,-0.55 0.12, 0.99, 0.03 0.54,-0.09, 0.84
P4 0754:11 153.2, 1.90,0.04 0.69, 0.24,-0.69 0.69,-0.50, 0.52 0.22, 0.83, 0.51
Table 1. Minimum Variance Analysis results. UT indicates instances of the center
of the positive Bz variations. MVA was performed over a variable window around the
specified UT. Results with the best ratio of λ2 and λ3 are shown.
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UT
a)
b)
P1
P2
P3
P4
P5
-4
-2
0
2
4Z
0 -5 -10 -15 -20X
Y
-4
-2
0
2
4
P3(THB)
P4(THC)
P5(THA)
P1(THB)
P2(THC)
c)
Figure 1.
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B,
nT
E,
mV
/mE
NE
RG
Y,
eV
i+
e-
N,
1/c
m3
V,
km
/sP,
nP
a
Vx
Vy
Vz
Pp
Pm
B,
nT
E,
mV
/mE
NE
RG
Y,
eVN
, 1
/cm
3V
, k
m/s
P, n
Pa
i+
e-
Vx
Vy
Vz
Pp
Pm
Ex
Ey
DSL
GSM
Ex
EyDSL
GSM
GSM
GSM
TH
EM
IS P
1 (
TH
B)
TH
EM
IS P
2 (
TH
C)
B,
nT
Bx, By, Bz
GSM
Bx, By, Bz
GSM
a)
b)
c) d)
P1
P2
Figure 2.
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B,
nT
E,
mV
/mE
NE
RG
Y,
eVN
, 1
/cm
3V
, k
m/s
P, n
Pa
Vx
Vy
Vz
Pp
Pm
i+
e-
B,
nT
E,
mV
/mE
NE
RG
Y,
eVN
, 1
/cm
3
Vx
Vy
Vz
Pp
Pm
V,
km
/sP,
nP
a
Ex
Ey
Ex
EyDSL
DSL
GSM
GSM
GSM
GSM
TH
EM
IS P
3 (
TH
D)
TH
EM
IS P
4 (
TH
E)
B,
nT
Bx, By, BzGSM
Bx, By, BzGSM
P3 P4
a)
b)
c) d)
Figure 3.
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RUNOV ET AL.: DIPOLARIZATION FRONT X - 19
B,
nT
E,
mV
/mE
NE
RG
Y,
eVN
, 1
/cm
3V
, k
m/s
P, n
Pa
Vx
Vy
Vz
Pp
Pm
DSL
GSM
GSM
TH
EM
IS P
5 (
TH
A)
Ex
Ey
MVA V_bulk
0751:260752:350752:58
0754:07
0754:10
b)
a)
Figure 4.
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